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Defender Viscous Fan


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G'day,

I'm having trouble with a Viscous fan on my '02 Defender. My fan is newish and operates as it should, until I use my A/C. About a minute or so after I turn on the A/C my fan will engage and remain so, even down the highway at 100k, which loads my engine and destroys fuel economy. But if I turn the A/C off the fan disengages after about 2 or 3 minutes. The condenser fan works as it should and I've flushed the main radiator and refilled her with proper coolant. I've also taken the intercooler off and given her a good clean. There are definitely no obstructions to air flow and the engine has never remotely overheated. Im at a complete loss.

Driving the outback with no A/C is a bit of a pain.

Any help or advice would be tops.

Ta.

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IIRC the viscous coupling locks up when the air flowing over its central hub is hot enough. Running the AC means the air flow exiting the radpack will be hotter both becuase the AC condenser itself is dumping heat and because the engines working harder, so it makes sense the fan could lock up by turning on the AC.

Its possible that when driving normally, the airflow temperature is just below the threshold, especially if the ambient temperature is very high, and the extra heat from the AC simply tips it over the edge.

I dont believe the fan can consume much more than about 1hp, so in the grand scheme of the power being used by a landrover shaped object hurtling down the road at 100kph the fan isnt using enough power on its own to make much of a dent in your fuel consumption.

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I think I saw somewhere that Air Con is something like an 8 HP drain on an engine...

Add in your already higher ambient temps, Air con sapping power / MPG, and both rad & condenser chucking out heat, I would expect fan to lock in.....

My MPG generally dives when I use Air con.... Something like 28 MPG averafe down to 25 MPG when air con on all the time in hotter parts of Europe etc.

I suspect Air con is your hit on MPG, not the viscous fan...

Neil

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Viscous fans use very little energy when the vehicle is moving as the blades have an insignificant or even a negative angle of attack. They only work hard at low speed, which is exactly what you need. It sounds like everything is functioning perfectly. Your fuel consumption is from the aircon, not the fan.

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Agree with the above, the AC compressor will be using far more fuel than the fan, especially when travelling at 100KMH, where the air speed produced by the fan will be similar to road speed.

I think if there are any gains to be had with electric over viscous, then it would be more in traffic and town driving.

It is possible that the viscous unit is locking up prematurely, but doesn't sound like it to me.

How do you know your fan 'engages'?

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Viscous fan usually shouldnt be locking at highway cruising speeds , I ran 300tdi 130 DCHCPU in OZ and with aircon on and climbing over the escarpment towing close to 4 ton you could hear the viscous engage and then about 2 mins later release. The temp guage would hardly move noticeably . If the temp guage is staying in normal, and you have checked condenser fan in not rotating wrong way ( not unheard of with sucker fan fitted in blower fan position) then it may be worth trying a different viscous unit , as it may be locking at too low a temp .

Should mention this was in summer in WA type ambient temps :)

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Viscous fan usually shouldnt be locking at highway cruising speeds , I ran 300tdi 130 DCHCPU in OZ and with aircon on and climbing over the escarpment towing close to 4 ton you could hear the viscous engage and then about 2 mins later release. The temp guage would hardly move noticeably . If the temp guage is staying in normal, and you have checked condenser fan in not rotating wrong way ( not unheard of with sucker fan fitted in blower fan position) then it may be worth trying a different viscous unit , as it may be locking at too low a temp .

Should mention this was in summer in WA type ambient temps :)

In my experience the airflow throw the Deefer engine bay is ****, it becomes a high pressure zone and so it needs that viscous fan to keep pushing the air out of the engine bay at speed, not necessarily draw the air through the rad.

This has been proven by quite a few here in Australia over many years.

The Tdi or TD5 would start to overheat at highway speeds, checked and replaced the fan clutch and the problem is solved.

A work around is to cut a slot or hole in the side of the guard (not the top, top, the side is a low pressure region, the air flow is really disturbed on top of the guard, it does strange things) and let the hot, high pressure air escape.

The condenser fan on the Oz spec A/C is next to useless, it's justa single Davies Craig 10" fan and only really helps at low speeds.

Again, IME it actually blocks flow at high speed, as does the Oz spec condenser, which has a ridiculously fine fin pitch to get capacity and blocks air flow to the rad and intercooler. The condenser is an aftermarket and off the shelf Sanden part, not something specific to the Defender here on those older models.

In winter I found up to 5* difference in coolant temp between the condenser in place and uninstalled.

And another FWIW, the stock temp gauge is garbage, it has a huge amount of latitude built into it's movement, it's incredibly innacurate and is unduly influenced by the usual Defender **** earths.

A decent aftermarket and preferably mechanical guage will show exactly what's going on, or better still install something like the Madman EMS or Engine Watchdog TM2

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I always found it ridiculous that aircon condensers on 4x4 specced for Australia were fitted in front of the rad , whenever working hard

climbing sand dunes etc all the 4x4 makes had to turn of the aircon as the vehicles would start to overheat , that was why I fitted a roof top unit on my 110 copying the practice of mining equipment and was able to enjoy the benefits of cool air when you most need it even with the V8 engine :)

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Most factory temperature gauges "lie" to some extent, by design.

The one in my Audi sits at exactly 90c at any temperature between about 80 and 100c, and its intentional, as the cluster knows the exact temperature and will provide it to you via its diagnostics system. And even when it climbs out of that buffer it doesnt move round to the correct value, so 75c will have the gauge showing maybe 85ish and similarly 105c is barely beyond 90. I had a water pump failure one day, driving out of a carpark noticed the gauge was pointing at about 93 rather than its usual 90, barely perceptible shift, but because it wasnt in its usual bang upright position i figured something wasnt right. Stopped the car and pulled up the live temperature on the diagnostics and it was just under 110c. Was lucky i'd glanced at the guage while sat at the lights!

The reason is simply that manufacturers dont want the gauge wandering around, it looks bad. People expect their engine to smoothly warm up then sit there at 90, so they program the gauge to do exactly that. On my track car i've modified the curve in the EEPROM of the instrument cluster to remove the buffer, and the gauge wanders around. Floor it out the pits and you see the gauge swing round a bit past 90 then drop back as the thermostat opens. After a few hot laps it will settle a smidge under 90 which is fine, but then lift off and have a cooldown lap and the gauge plummets down into the 70s and then slowly recovers as the thermostat responds to the change in load.

As for the condenser in front of the rad, perhaps the condenser better requires the cold air? The main rad is only trying to keep the engine at 90c so the pre-warmed air from the AC condenser will likely do a fine job. It may be that if you put the condenser behind the rad, the hot air off the radiator wouldnt provide sufficient cooling to keep the AC working properly? I dont think i've ever seen a factory car that didnt have the AC condenser up front... Likely means theres a reason for it!

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After years of trying various things the solution I came up with that actually works is I relocated the condenser and it's fans to the underside of the roof rack. I've now traveled all over Australia in all levels of temperature from -8* to 52*C and my radiator never ever goes above 90*C.

If you run the A/C your economy WILL suffer, mine (Disco 300Tdi) goes from about 12L/100K to around 16L/100K - it takes about 8 hp to run the A/C compressor when its on full demand for cooling - instead of running your A/C try winding down your windows and install a roof rack that completely shades your roof - as in a tropical roof - it works wonders in keeping the cabin, and you, cool.

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After reading your replies and scratching my head for a few hrs I took her for a drive into town (Darwin) in the hottest part of the day. A few k's down the road I turned on the A/C and after a minute or so at constant speed (80kp/h) I heard the Viscous kick in, so I drove round for a while letting the engine temp get up to max operating. Then on the way home I turned off the A/C and the Viscous disengaged after about another 2 minutes or so.

It has only started doing this for about 6 months or so with a fan change in that time so the only thing I can think of is a re-gas of the A/C as it's radiator is creating enough excess heat to engage the coil on my fan. I know a few blokes over hear who cut a free flowing exhaust panel behind the passenger front wheel to help with air flow but I'd rather not have to bust out the grinder just yet.

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Why not replace the drivers' side wing top vent blank plastic with the proper grille from a LHD vehicle heater intake? No cutting, and as far as I know, it's in a low pressure zone, so should help evacuate the warm air with no damage to the vehicle. The grill will be cheap enough... If you need more ventilation still, then using a snorkel that goes through the wing top will help - the snorkel will suck in cleaner air away from the engine bay, and you can use the side grille with the interior duct removed as another vent and fit the alternative version on the other side (200 Tdi was on the left, all later vehicles on the right). The newly fitted side grille would sit proud of the surface, but you could bond a flange to the inside of the wing to mount it flush.

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